The development of solid-state power amplifiers for RF transmitters has created challenges to designers not present in previous tube designs. One major problem with solid-state designs is their limited power handling capability. While high power devices have been developed, they are generally quite expensive and thus are not desirable for designs where cost is a significant factor.
One strategy for solving this dilemma has been to divide the signal to be amplified into several components and direct them to a like number of smaller solid-state power amplifiers. The outputs of the power amplifiers are then combined to provide an output signal level which is comparable to or higher than the output signal which could have been obtained from a single high power solid-state power amplifier.
This divide-and-conquer strategy has its own drawbacks, however. The primary drawback was that previous signal dividers and combiners had used conventional wound transformers and lumped inductive and capacitive components to achieve the required impedance matching. Such components are inherently narrow-banded and are thus impractical for applications where wide bandwidths are required. Modern solid-state power amplifiers are generally broad-banded, and conventional narrow-banded signal dividers and combiners severely limited their utility.
One solution to such narrow-banded dividers and combiner was provided by U.S. Pat. No. 4,774,481 to Edwards et al., which discloses a broadband non-directional signal combiner (non-directional meaning that the combiner can be used as either a combiner or a divider). The combiner utilizes coaxial cables interconnected in a bridge configuration, and a coaxial cable transformer. The bridge configuration increases bandwidth, while the transformer counteracts the impedance transforming characteristics of the combiner. The combiner disclosed by Edwards et al. also incorporated ferrite sleeves over each coaxial cable in the combiner to eliminate even mode impedances between the cable outer conductors and the common ground plane.
The resulting combiner disclosed by Edwards et al. combines and divides signals across a broad range of frequencies with relatively large isolation between input ports, and a low voltage standing wave ratio. However, the use of ferrite sleeves introduces core losses resulting in heat dissipation and degraded intermodulation distortion performance ("IMD") at high power levels, limiting its usefulness in some applications. In addition, the combiner has a relatively large number of interconnections which act as discontinuities in the circuit, which increase insertion losses.
What is still needed, therefore, is a non-directional signal combiner having a short signal path with few discontinuities, such that insertion losses are low and relatively little inductance is required in the signal path. What is also needed is a non-directional combiner that combines and divides signals across a broad range of frequencies with large isolation between input ports and a low voltage standing wave ratio. What is further needed is a non-directional combiner that inherently exhibits excellent IMD characteristics, such that high power can be handled with little distortion. Preferably, the combiner will also have a simple design, be conducive to mass production, and be rugged and durable yet relatively inexpensive.